Position of floating button

Any idea how to set the floating position so that it is dead center horizontally…
I use this script and although I can position the button exactly where I want it on my screen, on the phone it is in a different position especially portrait to landscape …

badge - 1


display: none;

.fab-target >:nth-child(1) {
position: fixed;
top: 62% !important;
right: 38% !important;


horizontally not a big problem… vertically is all about resolution and zoom,